Centrifugal systems of separation use centrifugal force generated through rotation to separate fluid components of differing densities. In many fundamental aspects, these systems are used as a substitute for and improvement on gravitational separation techniques and devices, since the gravitational force and the force exerted on a fluid through rotation (centrifugal) are identical in that they increase in magnitude as the fluid increases in mass. Those fluids with lesser density will be less influenced by the force and therefore less inclined toward the source of the force, the earth for gravitational, the outside of the rotating container for centrifugal, than fluids with greater density. The fluids will thus separate out and can be directed to separate collection ports by using weirs or other suitable separating structures. Centrifugal separation is often more desirable than gravitational because the force applied to the fluid can be controlled through rotation speed and can be made to be many times that of gravity.
A common example of fluid separation is that of oil from water. There are many situations in which separation of these two elements is desired, such as oil spills on an ocean or lake, mixing of the two fluids in ships' bilges, gasoline spills, etc. This process is often important for maintenance of quality of life in a particular geographic area. These two fluids are susceptible to centrifugal separation because water is denser than oil and thus will "sink" relative to the other under application of centrifugal force. This can easily be understood by the fact that oil floats on water in a gravitational field. Other fluid separation applications include wine clarification, waste-water treatment, blood plasma separation, and the like. Centrifugation is also used to separate solids out of liquids through sedimentation.
It is often desirable to separate dissolved elements in solution or emulsion. Standard centrifugal separation equipment alone cannot carry out such a separation since the dissolved elements will move with the solution. A solvent must therefore be introduced into the fluid stream to extract the dissolved elements. Such a process requires that the solvent be thoroughly mixed with the fluid to extract all dissolved elements. The solvent and fluid are then separated through centrifugation. An example of this type of separation is solvent extraction and separation of transuranic elements from radioactive waste streams at nuclear processing plants.
Meikrantz, U.S. Pat. No. 4,959,158, is an example of a typical centrifugal separator in the prior art. In that apparatus, the fluid to be separated is introduced into a space between an inner rotor and an outer stationary housing, where shear mixing of the fluid occurs. A large amount of power is required to maintain the speed of the rotor against the viscous drag of the shearing liquid which makes the apparatus energy inefficient. The power loss increases with angular velocity, limiting the rotor speed.
The rotor is an open top cylinder with the separating weirs at the top. Meikrantz uses the space between the rotor and its housing to introduce the oil-water mixture to be separated. Thus the bottom portion of the space is filled with liquid during operation. The top portion of the space, where the liquids separate and transfer from the rotor to the housing, must contain air for proper operation. (This requires the separator to operate in an upright position). In this space, no seals can be made between the incoming liquid and the air because of the large diameter of the interface; the drag would be unacceptable. This highly agitated air/liquid interface causes the fluid input into the rotor to mix with the air, reducing flow capacity and causing foaming with many substances such as detergents in motor oil. This foaming dramatically reduces the effectiveness of the separator and further increases viscous drag. Meikrantz allows a second air-water interface to exist within the rotor, as a core of air forms radially inward from the first weir, at the center of the rotor. Energetic flow of liquid through the separator causes surface waves to form on this interface, which further degrades the separation process. Additionally, the lighter separated liquid spreads along the full length of the rotor between the air core and the partially defined liquid/liquid interface. This disperse unstable mass is difficult to collect over the relatively short first weir.